With respect to the synthesis of organohalosilanes, Rochow first disclosed in U.S. Pat. No. 2,380,995 direct synthesis reaction between metallic silicon and organohalide in the presence of a copper catalyst. Since then, there have been reported a number of research works relating to copper catalysts and treatment thereof, co-catalysts used together with copper catalysts, reactors, additives used during reaction, and the like. In the industrial synthesis of organohalosilanes, the selectivity of diorganodihalosilane which is most widely used in silicone resins, the formation rate of silanes, and the percent conversion of metallic silicon into useful silane are crucial. The selectivity of diorganodihalosilane is evaluated in terms of a weight or molar ratio of dialkyldihalosilane to the silanes produced and a T/D ratio. Organohalosilane products contain diorganodihalosilane (D), triorganohalosilane (M), organotrihalosilane (T), etc. as well as other by-products such as organohydrodihalosilane (H) and organohalodisilanes. Particularly when silicones are manufactured using the direct method organohalosilane product as a starting material, few processes are available for the effective utilization of disilanes known as a high-boiling fraction. Thus, most disilanes are discarded as the residue.
The T/D ratio is a compositional ratio of organotrihalosilane to diorganodihalosilane in the entire organohalosilanes produced, with a lower T/D ratio being preferred. The formation rate of organohalosilane is represented by a space time yield (STY) which is the weight of crude organohalosilane produced per unit time relative to the weight of metallic silicon held in the reactor. In order to improve the content of diorganodihalosilane produced, reduce the T/D ratio or increase the STY, various research works have been made with a focus on the catalyst and promoter.
U.S.S.R. Application Specification No. 1,152,943 (Certificate of inventorship No. 237,892) dated Nov. 20, 1969 discloses to add a phosphorus-copper-silicon alloy to a contact mass so as to give 2,500 to 30,000 ppm of phosphorus, thereby improving the dimethyldichlorosilane content to 82.3%. Use of a promoter-containing metallic silicon alloy is not adequate to industrial scale reaction, and the STY and silane composition are unsatisfactory. U.S. Pat. No. 4,602,101 corresponding to JP-B 5-51596 discloses that 25 to 2,500 ppm of a phosphorus compound capable of generating elemental phosphorus in the reactor is added to a contact mass. Although the results of reaction according to this U.S. patent are improved over the U.S.S.R. patent, there still remain many problems including hazard imposed by spontaneously igniting elemental phosphorus and increased cost of raw materials. Then this U.S. patent is also unsuitable to apply to commercial scale reactors. There is no teaching about the effect available with a phosphorus concentration of at least 2,500 ppm. Also, F. Komitsky et al., Silicon For the Chemical Industry IV, Geiranger, Norway (1998), page 217, proposes the addition of phosphorus in the form of copper phosphide, leaving problems including a low percent conversion, ineffective utilization of phosphorus, and difficult control of a phosphorus concentration.
JP-A 11-228580 or EP 893408 (Pecine Electrometallurgy) discloses that active silicon powder is prepared by milling metallic silicon in a rod mill or ball mill using rods or balls of bronze phosphide whereby bronze phosphide finely adheres to surfaces of silicon particles, rendering the silicon particles active. The bronze phosphide used therein is ordinary bronze phosphide having a phosphorus concentration of up to 1% by weight. Phosphorus is added in the form of iron phosphide. This gives rise to essentially the same problem as the prior art proposals.